This invention relates to mounting arrangements and methods for mounting an element on a rigid substrate which in use is subject to high strain.
In particular, but not exclusively, this invention is concerned with mounting an acoustic transducer component on a barrier to achieve Non Penetrating Data Transfer (NPDT) across a barrier which in use is subject to high strain. In this specification, the term ‘high strain’ is used to mean strains in excess of 0.5×10−3. The invention however also is applicable in other situations where an element (such as a sensor) is mounted on a substrate subject to high strain and needs to be protected against the high strain either to prevent damage to the element itself or to a fixing interface such as an adhesive layer, between the element and the substrate. Such elements include sensors such as, for example, thermocouples.
NPDT is a technology that permits the transmission of data and power through solid barriers using ultrasonic acoustic waves. NPDT can reduce or eliminate the traditional penetrations required for wired connections through protective barriers. This is highly attractive when the structural integrity of the barrier, or its hermetic integrity, is of paramount importance; for example in submarines, the nuclear industry and the chemical industry. Reducing the total number of penetrations through such barriers may enhance safety whilst reducing installation costs, and through-life costs. This technology can also enable the retrofitting of new sensor capabilities to existing platforms to meet emerging requirements without compromising the barrier's performance. For examples of such methods reference is directed to our earlier patent publication EP2122868, and co-pending applications PCT/GB2010/051469 and PCT/GB2010/051470, the contents of which are incorporated here and by reference.
In particular applications data is transmitted though a solid barrier using high frequency acoustic signals typically, but not exclusively, in the frequency range ˜3 MHz to ˜55 MHz. Piezo-electric transducers bonded onto opposite surfaces of a solid barrier by a very thin adhesive layer launch or receive these ultrasonic signals. The ultrasonic beam launched by each transducer is highly directional and will remain collimated over typical barrier thicknesses likely to be encountered in the real world. Digital data transmission rates of >150 MB/s are viable through a single acoustic channel link, and separately power transfer of up to ˜100 watts has been successfully demonstrated. Although we have used adhesives in our current work (as referenced above) to bond the transducer to the substrate or to the carrier plate, other materials could be used subject to not cracking the transducers due to thermal shock during the bonding process by using, for example a low temperature solder, or other bonding materials.
In existing designs, the Piezo-electric transducers of the NPDT links need to be bonded either directly onto the barrier, or alternatively onto a thin carrier plate which in turn is bonded or mounted onto the barrier. In naval applications the barrier may, for example, be made from high tensile naval steel. Although direct mounting of the transducer element on the substrate is possible, for wide frequency bandwidth data transducers there are significant benefits gained by mounting the transducer on an intermediate plate, and in specific cases a plate of wedge section. The mean thickness directly below the active Piezo-electric transducer element of such plates is typically 3 mm or so. Where, as is usual, a non-compliant bonding/coupling layer is used, it is understood that this plate should be made as thin as possible to minimise the shear forces on the bond to the barrier in response to contraction due to the mechanical stiffness of the plate.
In NPDT, good data or power transfer may be achieved using good acoustic coupling between the transducer, the substrate and the other transducer. High frequency acoustic transducers can be acoustically matched to the substrate in order to efficiently couple the acoustic energy in and out of the substrate by reducing acoustic reflections at the interface between each transducer and the substrate.
Acoustic matching is determined in part by the ratio of the acoustic impedances of the respective materials that are bonded together. The acoustic impedance Za of a material is equal to the product of its density ρ and acoustic velocity va. The acoustic reflectivity Rinterface of an interface between two materials of acoustic impedance Z1 and Z2 is given by the formula:
                              R          interface                =                                            (                                                Z                  2                                -                                  Z                  1                                            )                        2                                              (                                                Z                  2                                +                                  Z                  1                                            )                        2                                              (        1        )            
The acoustic reflectivity of a hypothetical interface between a 36° Y cut lithium niobate transducer of acoustic impedance 34.3 MRayls (1 MRayls=1×106 kg m−2 s−1) and a naval steel substrate of acoustic impedance 46.3 MRayls would be ˜2.2%, which is very low, and therefore these materials would be considered acoustically well matched.
Meanwhile the acoustic reflectivity Rbond of a pair of interfaces formed by an acoustic bonding layer between two materials is given by the formula:
                              R          bond                =                                            (                                                Z                  bond                                -                                  Z                  1                                            )                        2                                              (                                                Z                  bond                                +                                  Z                  1                                            )                        2                                              (        2        )            
Where the acoustic impedance Zbond arising from the finite thickness L of the bond layer, valid for low levels of acoustic attenuation within the bond layer, is given by:
                              Z          bond                =                              Z            2                    ⁢                      {                                                                                Z                    3                                    ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                                                        k                          2                                                ⁢                        L                                            )                                                                      -                                                      iZ                    1                                    ⁢                                      sin                    ⁡                                          (                                                                        k                          2                                                ⁢                        L                                            )                                                                                                                                        Z                    2                                    ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                                                        k                          2                                                ⁢                        L                                            )                                                                      +                                                      iZ                    3                                    ⁢                                      sin                    ⁡                                          (                                                                        k                          2                                                ⁢                        L                                            )                                                                                            }                                              (        3        )            
Here Z1 and Z3 are the acoustic impedances for layers 1 and 3 on both sides of the bond line and Z2 is the acoustic impedance of the bond layer of thickness L, while k2 is the acoustic k vector given by 2π/Λ for sound of wavelength Λ in side the bond material.
For the specific case of a 36° Y cut lithium niobate transducer bonded to a steel substrate it is found that the frequency bandwidth response of the transducer becomes severely compromised once the reflectivity of the transducer-substrate bond interface exceeds ˜25% to ˜30%.
Adhesive bonding is commonly used for materials such as steel or glass. However, the adhesive materials are generally poorly matched to the transducer or substrate material, for example the adhesive EP30 supplied by MasterBond has an acoustic impedance of only 1.97 MRayls which is a factor of 23 smaller than that of Naval Steel. This can result in a very narrow fractional bandwidth Δf/fc performance, where Δf is the frequency bandwidth for efficient acoustic transduction, and fc, is the centre frequency of operation for the transducer. This is normally overcome by using a very thin bond of under 1/100th of a wavelength in thickness so, for example, for a 40 MHz centre frequency transducer this would be about 0.5 μm. If this thickness criterion is met, then fractional bandwidths of greater than ˜30% to ˜50% become possible depending on the specific transducer and substrate design combination. But in applications where very high compression strains are seen by the transducer such as on parts attached to a submerged submarine hull, when it deep dives, or in high pressure oil pipes, the compressive strain (or tensile strain in high pressure oil pipes) can lead to delamination or damage or bond failure to the transducer thereby impairing, or preventing data transfer.
Presently accepted design considerations therefore suggest that the thickness of any carrier plate should be as thin as possible to reduce shear stress on any adhesive bond layer to the barrier, and likewise any such adhesive layer should be as thin as possible so that reflections of acoustic power at the interface to the barrier are minimised. Our studies have however shown that the problems of delamination can be significantly reduced by attaching, e.g. by welding, a relatively thick element between the transducer and the barrier. This reduces the strain at the critical bond interface of the transducer to this thick element, or in the case of a transducer mounted on a carrier plate the critical bond interfaces between the transducer and the carrier and the carrier and the thick element.